Learn why fluorescent light bulbs are an efficient light source and a great option for lighting your home. This article examines the mechanics behind fluorescent lighting, and identifies the fluorescent lighting options available for residential lighting.
Fluorescent bulbs consist of a tube of glass with one electrode on each end. The tube is filled with a very low pressure of argon and a drop of mercury. The inside surface of the tube has a coating of finely sized phosphors. The tube’s diameter is measured in eights of an inch. Thus, a T8 is one inch (8 eights of an inch) in diameter.
The fill pressure inside the tube is only about 1/750 that of atmospheric pressure. Given the low pressure (vacuum) inside, these bulbs make an impressive implosion when they break.
Since all fluorescent bulbs contain mercury, breaking the bulbs should be avoided, and the bulbs disposed of in accordance with local environmental protection laws.
A fluorescent bulb needs a much higher voltage than what is available at a wall outlet, and also needs some way to control the amount of current flowing through it. These two functions are filled by a circuit called a “ballast.”
There are two main types of ballasts: the older magnetic ballasts, and the newer electronic ballasts. The electronic ballasts are more expensive, but are significantly more efficient.
In working out the life cycle costs (LCC), also keep in mind that although the ballasts last much longer than the bulbs, they won’t last forever.
How Fluorescent Lamps Produce Light
Fluorescent lamps fall into the category of “discharge” sources. The light is produced by electrically energizing a low-pressure gap between two electrodes.
In fluorescent bulbs, the electrodes are similar to the filaments used in incandescent bulbs, except that they have a coating (called an ’emission mix’) that increases their ability to emit electrons. As with incandescent bulbs, the tungsten electrodes evaporate with time, so the ends of a fluorescent tube tend to blacken.
As high velocity electrons flow through the gap, they collide with the gaseous atoms in the tube (argon and mercury vapour atoms). The collision between an electron and an atom of mercury imparts energy to one of the electrons in the atoms. The atom does not like being in this higher state of energy, so the atom’s electron promptly falls back to its original lower energy state. As it does so, a very precise amount of energy is released from the atom. This released energy takes the form of a photon of ultraviolet light.
Although argon is present in the tube, it emits a dim purplish color that is not very useful. The dominant source of the light being emitted comes from the mercury atoms, and the radiation is invisible ultraviolet at first. The phosphors on the tube wall absorb the invisible ultraviolet and emit other visible colors instead.
Many fluorescent bulbs contain three types of phosphors (approximately red, green, and blue), which together give the illusion of white light, similar to the behavior of the phosphors inside your bulky CRT computer monitor.
The Importance of Color Temperature and CRI
Not all fluorescent tubes are created equally. Some have a pleasing color, others look hideously greenish. Two numbers are important to the color: The color temperature and the color rendering index (CRI). A color temperature around 2800K will give a color that has the warm, reddish hue of an incandescent bulb. A 4100K is cool white. Look for a CRI of at least 80, or expect some colors in the room to not look right.
Fluorescent bulbs are highly efficient, around 80 LPW (lumens per watt), compared to only about 10 or 12 LPW for incandescent.
Fluorescent bulbs also last a long time, typically 10,000 to 20,000 hours. The life is strongly influenced by the number of times the bulb is turned off and on. The more often it is turned off and on, the shorter the life.
As mentioned earlier, another factor is that electronic ballasts are more efficient than magnetic ones.
Another factor is the diameter of the bulb. This is a case where technological advances can run contrary to intuition.
Since a larger diameter bulb provides a larger surface area for the phosphors, a smaller diameter tube would be expected to produce less light. Yet the slimmer tubes and their ballasts often are engineered to produce more lumens.
An indirect advantage to slimmer bulbs is that the fixtures that hold the bulbs can be designed with more efficient reflectors. With a fat T12 bulb, it is difficult to make use of all of the lumens. The latest T5 bulbs offer significant optical advantages. A well designed reflector behind a T5 will put more lumens down on the work surface than a T8 or a T12 system.
Some of the latest bulb/ballast technologies are claiming a life of 30,000 hours, which is twice the life that fluorescents had only 15 years ago. The longer-life systems, of course, cost more, so the customer must consider the life cycle costs.
One technological advance that is often not worth paying extra for is increased lumen maintenance. All fluorescent bulbs get dimmer as they age, but the human eye doesn’t notice it. If I have bulbs with 90% lumen maintenance (near the end of life they have 90% of their initial brightness), then I would not pay extra for bulbs that have 95% lumen maintenance. If the drop in light output is less than 20%, the human eye can’t discern the difference anyway. The difference between 90% and 95% lumen maintenance is only 5%, and that is not a discernible difference.
The disadvantages are the large size of the lamp, the large initial cost for a large fixture and ballast, and a higher replacement bulb cost than for incandescent.
Again, do the math. If the lamp is going to see many thousands of hours of service, the total cost (including the electricity) is much lower for fluorescent than for incandescent or halogen.
The Effect of Temperature on Fluorescent Bulbs
Note that the brightness of fluorescent bulbs is highly dependent on the temperature of the bulb, and the coldest spot on the bulb determines the brightness for the entire bulb.
Fluorescent bulbs reach their maximum light output when the coldest spot on the bulb is about 77F. As the temperature decreases, the mercury vapor condenses, so there are fewer and fewer mercury atoms in the arc. Thus the bulb gets dimmer.
By the time you reach 32F, the bulb produces very little light. This is an important factor if you are considering installing fluorescent bulbs in an unheated garage or shed.
As the temperature rises above 77F, the number of mercury atoms in the arc increases. Intuitively that would be a good thing, but it isn’t. If there are too many mercury atoms, then the ultraviolet emitted by one mercury atom collides with another mercury atom rather than colliding with a phosphor. As the temperature rises above 77F, the bulb gets dimmer.
The life of a fluorescent lamp depends upon the condition of the electrodes (which degrade with time), and on the availability of mercury vapor.
Fluorescent bulbs have been around since the 1940s, but no one has proven for certain all what happens to the mercury and why. The mercury is still in the bulb, but it gets “tied up” with the phosphors and the glass over time, so there is less and less available as a vapor.
Initially, bulb manufacturers compensated by putting a lot of mercury into the bulb. These days, disposing of the mercury-laden bulb can be difficult and/or expensive depending on local laws. Environmental concerns have forced bulb manufacturers to reduce the mercury. But the mercury can’t be eliminated or the bulb won’t work.
Rapid Start Bulbs and Ballasts
There are different kinds of bulbs and ballasts, depending on how rapidly the bulb will start. If you use an ordinary fluorescent bulb in a rapid start ballast, the bulb will have a short life. Use rapid start bulbs with rapid start ballasts.
Thanks to our guest lighting expert – Lance Kaczorowski, who brings a wealth of expertise to the site:
Kaczorowski, a native of New York City, now resides in Fort Wayne, IN. Kaczorowski has a 4-year degree in Mechanical Engineering from the University of Texas at Austin, and also a 2-year degree in Electronics Engineering Technology from the Community College of the Air Force. Kaczorowski’s broad work history includes (chronologically): Three years as a Mercedes-Benz mechanic; six years as an electronics technician with the Air Force; three years as a new product development engineer with General Electric Lighting in Cleveland; seven years as a new product development engineer and an engineering analyst with Grote Industries in Madison, IN; and currently as an engineering analyst with International Truck and Engine Corporation in Fort Wayne.
The first two years of Kaczorowski’s employment with General Electric consisted of extensive training in light source sciences and engineering under GE’s Edison Engineering Program. Kaczorowski’s experience with lighting was broadened at Grote Industries, which is a supplier of vehicle lighting for heavy duty trucks.